Code division multiple access mobile communication system

ABSTRACT

A code division multiple access (CDMA) mobile communication system with improvements to permit stable reception with a minimum of bit error. The system comprises a voltage-controlled oscillator for supplying a carrier to a radio frequency quadrature demodulator, and a frequency controller for detecting a frequency error from a phase correction signal of the first step to generate a control signal that controls the oscillator. The frequency controller includes an extracting circuit and an integrating circuit. The extracting circuit extracts a phase change based on the frequency error derived from the phase correction signal of the first step and from a signal preceding that signal by a predetermined delay time. The integrating circuit integrates the phase change and outputs the integrated result as the control signal. The predetermined delay time is preferably set within a range not exceeding the delay time needed for averaging by an averaging circuit that receives the phase correction signal of the first step and outputs a phase correction signal. The carrier to a radio frequency quadrature modulator is preferably supplied from the voltage-controlled oscillator.

This application is a continuation of U.S. patent application Ser. No.09/299,101 filed on Apr. 26, 1999, now U.S. Pat. No. 6,292,477 which isa continuation of U.S. patent application Ser. No. 08/709,734 filed onSep. 9, 1996 (now U.S. Pat. No. 5,943,329).

BACKGROUND OF THE INVENTION

The present invention relates to a mobile communication system operatingon what is known as the code division multiple access (CDMA) system.

The CDMA system involves multiplexing a plurality of communicationchannels using spread spectrum codes, each channel being assigned adifferent spread spectrum code. A given signal to be transmitted ismultiplied (i.e., spread) by the spread code assigned to the ownchannel, and is multiplexed with other similarly spread signals ondifferent channels before being transmitted. At a receiver, themultiplexed signals are multiplied (i.e., despread) by the same spreadcode so that only the target signal will be extracted correlated on theown channel. The signals on the other channels are perceived merely asnoise because these signals with their different spread codes remainuncorrelated. The level of the noise may be sufficiently lowered so asnot to disturb the signal reception. The CDMA system is attractingattention as a system fit for drastically improving the efficiency offrequency utilization and has been commercialized in some areas.

Where CDMA communication is implemented using spread codes, some kind ofsignal modulation (e.g., quadrature phase shift keying or QPSK) precedesthe spreading of the signal for transmission. At a receiving point, thedespreading of the signal is followed by demodulation. Despreading anddemodulation both represent the detection process whereby thetransmitted signal is reconstructed. Commonly used detection methodsinclude a coherent detection method based on the PLL (phase locked loop)circuit and a differential detection method. There also exists arecently proposed coherent detection method that utilizes pilot signals.

Where the CDMA system is applied to a mobile communication systemadopting the conventional coherent detection method, the bit error rateof data in a mobile station deteriorates if a fading occurs while thestation is moving. In a CDMA mobile communication system utilizing thedifferential detection method, the bit error rate of data in a mobilestation can worsen due to the noise on the air transmission channel evenif the station is stationary. The pilot signal-based coherent detectionmethod has been proposed for a system to minimize the deterioration ofthe bit error rate whether the mobile station is in motion or at rest.The method was discussed at the Autumn 1994 Symposium of the Instituteof Electronics, Information and Communication Engineers of Japan asdisclosed in the IEICE collection of papers B-5 on radio communicationsystems A and B, p. 306, “Coherent detection for CDMA MobileCommunication Systems” by Yasuo Ohgoshi et al.

Described below is a conventional mobile communication system that usespilot signals with reference to the above-cited paper supplemented bysome details. The description will first center on the down link of thesystem (i.e., a link from the base station to a mobile station). FIG. 13shows a modulation circuit 51 of a base station 1 that transmits dataand a first half 52 of the detection circuit of a mobile station 2. Thebase station 1 actually transmits signals to a plurality of mobilestations 2, and FIG. 13 shows one station as the representative example.

In the modulation circuit 51 (left-hand half of FIG. 13), data firstundergoes QPSK modulation, not shown, to divide into an in-phase signalI and a quadrature signal Q. The signals I and Q are spread (i.e.,multiplied) respectively by spread code signals PN_(−I D) and PN_(−Q D).The two spread code signals are supplied from a spread code generator91. The rates of the spread code signals PN_(−I D) and PN_(−Q D) (calledthe chip rates) are used to multiply by k (k: spreading ratio) thepre-spread rates (called the symbol rates) of the signals I and Q sothat the latter will attain the chip rates. The signals thus spread passthrough a radio frequency quadrature modulator 54 to become mutuallyperpendicular signals that are transmitted on a radio frequency bandfrom an antenna. A temperature compensated crystal oscillator 61 isprovided to furnish the modulator 54 with a carrier C_(B).

The pilot signals will now be described. The transmission circuit issubstantially the same as the left-hand half of FIG. 13 and is omitted.An in-phase signal I_(P) and a quadrature signal Q_(P) of the pilotsignals are spread respectively by spread code signals PN_(−I P) andPN_(−Q P). Both spread code signals have the same chip rate as in thecase of data. The pilot signals thus spread are subject to radiofrequency quadrature modulation by the same carrier C_(B) as with data,turning into mutually perpendicular signals transmitted on the sameradio frequency band as with data. The pilot signals serve as referencesignals for demodulation and are common to all channels utilized.

In the first half 52 (right-hand half of FIG. 13) of the detectioncircuit of the mobile station 2, the received signals from the antenna(data and the pilot signals) pass through a radio frequency quadraturedemodulator 57 to reach a low-pass filter 56. The low-pass filter 56removes the radio frequency components from the signals to yield signalsS_(I) and S_(Q). A crystal oscillator 60 supplies the demodulator 57with a carrier C_(M). The signals S_(I) and S_(Q) are composed of thespread signals I and Q (those destined to the own channel as well as toother channels) and of the spread pilot signals I_(P) and Q_(P). Assuch, the signals S_(I) and S_(Q) include a phase error caused by fadingand a frequency error attributable to the precision of the oscillator60.

The errors included in the signals S_(I) and S_(Q) produce a phasedifference therein. When the mutually perpendicular pilot signals areplotted in orthogonal coordinates, the received pilot signals arerotated exactly by the phase shift, as shown in FIG. 14. If the phaseshift is represented by φ and the orthogonal coordinates afterquadrature demodulation are designated by X₁ and Y₁, then the coordinateaxes X and Y of the received signals are rotated by φ displacing thepilot signals. Consequently, the undisplaced signals i and q that shouldhave resulted with no phase shift become i₁ and q₁ respectively. Suchchanges are caused by the mixing of one of the two mutuallyperpendicular signals into the other signal. The phenomenon is expressedby the following formulas:

i₁=i cos φ−q sin φ

q₁=q cos φ+i sin φ

The pilot signals are signals that stay constant following thedespreading. Generally, i=1 and q=1. The signal changes into i₁ and q₁permit acquisition of a signal CS with the value cos φ and a signal SNwith the value sin φ. With the two signals known, it is possible tocorrect the phase rotation of the data. Since the data includes the samephase shift, the despread data signals are inversely rotated by φ usingthe signals CS and SN whereby the initial signals I and Q are correctlyreconstructed. Thus the signals CS and SN serve as phase correctionsignals.

The signals S_(I) and S_(Q) output by the first half 52 of the detectioncircuit are subject to despreading and phase correction by the secondhalf of the detection circuit shown in FIG. 15. A pilot signaldespreading unit 21 in the upper left portion of FIG. 15 despreads thesignals S_(I) and S_(Q) by use of the spread code signals PN_(−I P) andPN_(−Q P) from a spread code generator 25, whereby the pilot signals areextracted. The extracted pilot signals are then added and subtractedmutually, becoming a signal CS_(C) with a chip rate of cost and a signalSN_(c) with a chip rate of sin φ. The two signals are converted to thesymbol rates by an accumulator 41 and thereby turn into phase correctionsignals CS_(S) and SN_(S) of the preliminary stage. The phase correctionsignals are averaged by an averaging circuit 43 for noise reduction. Theaveraging provides the phase correction signals CS and SN of the finalstage.

FIG. 16 shows a typical circuit constitution of the averaging circuit43. Reference numerals 430 through 433 are delay gates (Ds) for delayinga signal by a one-symbol period each. In this example, three consecutivesymbol values are averaged when added up by adders 235 and 236. It isthrough this noise reduction arrangement that the phase correctionsignals CS and SN are obtained. The delay time (average delay time) Trequired for the averaging by the averaging circuit 43 is given as

T=Ds×(N−1)/2

where N denotes the number of symbols used for the averaging operation.

The data signals S_(I) and S_(Q) are both despreads by an inverse dataspreading unit 42 (bottom left in FIG. 15) using the spread code signalPN_(−I D) for the signal I and the spread code signal PN_(−Q D) for thesignal Q. The operation causes four signals to be extracted. The fourchip rate signals are converted by an accumulator 44 into symbol ratesto become signals D₁ through D₄. After this, the signals D₁ through D₄are each delayed by a data delaying unit 48 (FIG. 17) by the averagedelay time T of the averaging circuit 43. The operation yields signalsD₁ ₀ through D₄ ₀. Where the data delaying unit 48 is constituted by anumber of delay gates (Ds) in stages of cascade connection each gateproviding one-symbol period delay, the gate count M per stage is givenas

M=(N−1)/2

In the above example, N=3 and thus M=1, so that the delay gates 480through 483 of the data delaying unit 48 are each composed of aone-symbol delay gate (Ds).

The signals D₁ ₀ through D₄ ₀ are fed to a phase correction circuit 49in which the signals are corrected in phase rotation by use of thecorrection signals CS and SN. A typical constitution of the phasecorrection circuit 49 is shown in FIG. 18. The phase correction circuit49 performs phase correction as follows: the signals D₁ ₀ and D₄ ₀ aremultiplied by the correction signal CS, and the signals D₂ ₀ and D₃ ₀ bythe correction signal SN. The multiplied results are added andsubtracted mutually so as to rotate the orthogonal axes of the receiveddata by −φ in phase (i.e., the phase shift φ is reduced to zero in FIG.14). The phase correction provides reconstructed signals I_(R) and Q_(R)of the original signals I and Q. The signals I_(R) and Q_(R) thenundergo QPSK demodulation, not shown, to become the original data.

One disadvantage of the conventional detection circuit above is that therestored signals I_(R) and Q_(R) are unavoidably affected by thefrequency precision of the crystal oscillator 60 (right-hand side inFIG. 13). A transmitter 60 used in the mobile station necessarilyincludes a certain practical frequency error because the mobile stationis for use by general users. That is, on the one hand, if the frequencyerror involved in the data is large enough to cause apparent phaseirregularities over the average delay time T during data demodulation,no precise correction signals can be acquired and the bit error rate ofthe detected data worsens. On the other hand, if the average delay timeT is shortened to avert the deterioration of the bit error rate, theadverse effects of the frequency error are diminished but the line noisebecomes more pronounced.

On the up link (i.e., a link from the mobile station to the basestation), the carrier C_(M) from the crystal oscillator 60 often doublesas a carrier for use in radio frequency quadrature modulation by themodulation circuit of the mobile station. In that case, the signalstransmitted by the mobile station and received by the base stationinclude both the phase error caused by fading and the frequency errororiginating from the crystal oscillator. The frequency error results inthe inevitable deterioration of the bit error rate in the detectionprocess of the base station.

The deficiencies above are conventionally circumvented, particularlywhere data of lower bit rates than normal are transmitted, by the methodof burst data transmission with no change in the spreading ratio, asstipulated by the U.S. digital radio communication standard IS (InterimStandard)-95. Under the system, transmitting data at 1/r of the standardbit rate compresses the data to 1/r in temporal terms. Thetime-compressed data is transmitted in bursts at fixed intervals.

How the burst signals are sent intermittently is illustrated in FIG. 19.In FIG. 19, the axis of abscissa represents time and the axis ofordinate denotes transmission power. Reference numeral 140 is a radiosignal waveform of standard bit rate data. Reference numerals 141, 142and 143 stand respectively for radio signal waveforms of data at{fraction (1/2, 1/4)} and ⅛ of the standard bit rate. The number ofburst signals varies with the bit rate. All burst signals have the samestandard bit rate when temporally compressed as described. It followsthat every burst signal has the same symbol rate and thus the spreadingratio remains unchanged.

The arrangements above are necessitated by the following reasons: ifcompression is not carried out, the one-symbol period gets longer thelower the data rate. Meanwhile, the number of symbols N for use by theaveraging circuit 43 (FIG. 15) in the demodulation circuit remainssubstantially the same regardless of the bit rate in view of noisereduction. Thus the average delay time T becomes longer the lower thedata rate. A prolonged delay time T prompts the frequency error todeteriorate the bit error rate as discussed above. The lower the bitrate, the more deteriorated the bit error rate. To avoid this deficiencyrequires keeping the symbol rate constant. The requirement necessitatesthe use of complicated circuits in the mobile station, which runscounter to the inherent need for the mobile station to simplify itscircuitry.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome the aboveand other deficiencies and disadvantage of the prior art and to providean improved CDMA mobile communication system permitting stable signalreception with a minimum of bit error.

In carrying out the invention and according to one aspect thereof, thereis provided a CDMA mobile communication system including a mobilestation comprising a voltage-controlled oscillator and a frequencycontroller. The voltage-controlled oscillator acts as a circuit tosupply a carrier to a radio frequency quadrature demodulator. Thefrequency controller detects a frequency error from a phase correctionsignal of the first step and uses the detected frequency error as thebasis for generating a control signal for use by the oscillator. Thefrequency controller may illustratively be composed of two circuits: acircuit for detecting a phase change caused by the frequency errorderived from the phase correction signal of the first step and from asignal preceding the correction signal by a predetermined delay time;and an integrating circuit for integrating the phase change andoutputting the result as the above control signal.

The voltage-controlled oscillator and the frequency controller operateto establish within the detection circuit of the mobile station acontrol loop whereby the phase change is reduced substantially to zero.This minimizes the frequency error. Because the frequency of theoscillator is kept as precise as that of the oscillator of the basestation, the phase shift attributable to the frequency error issignificantly reduced. This provides a detection circuit that worksstably with a minimum of bit error.

The predetermined delay time may preferably be set within a range notexceeding the delay time needed for the averaging operation by theaveraging circuit which admits the phase correction signal of the firststep and outputs a phase correction signal.

The carrier to a radio frequency quadrature modulator may preferably besupplied by the voltage-controlled oscillator. Because the frequency ofthe radio signal sent to the base station is kept accurate, the basestation is allowed to implement pilot signal-based coherent detectionstably with a minimum of bit error. Where the mobile station transmitsdata at a low bit rate, the system allows the terminal to keep the chiprate of the spread code constant and to transmit data with varyingspreading ratios but without time compression. Such data transmission isreadily implemented by changing the circuit constant in keeping with thesymbol rate of the data, with no change in the circuit constitution.

These and other objects and many of the attendant advantages of theinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a CDMA mobile communication system basedon a CDMA detection circuit and practiced as a first embodiment of theinvention;

FIG. 2 is a schematic view showing a pilot signal as it is related todata in a signal transmitted by the base station of the firstembodiment;

FIG. 3 is a circuit diagram of the second half of the detection circuitin a mobile station of the first embodiment:

FIG. 4 is a circuit diagram of a frequency controller used by the firstembodiment;

FIG. 5 is a circuit diagram of another frequency controller for use bythe first embodiment;

FIG. 6 is a schematic view depicting a pilot signal as it is related todata in a signal transmitted by the mobile station of the firstembodiment;

FIG. 7 is a circuit diagram of the second half of the detection circuitin the base station of the first embodiment;

FIG. 8 is a schematic view illustrating pilot signals as they arerelated to data in a signal transmitted by the mobile station of thefirst embodiment;

FIG. 9 is a schematic view explaining data transmission by the mobilestation of the first embodiment;

FIG. 10 is a schematic view showing a pilot signal as it is related todata in a signal transmitted by a mobile station of a second embodiment;

FIG. 11 is a circuit diagram of the second half of the detection circuitin the base station of a third embodiment;

FIG. 12 is a circuit diagram of a temporary judge circuit in the secondhalf of the detection circuit shown in FIG. 11;

FIG. 13 is a circuit diagram of the modulation circuit in a conventionalbase station and the first half of the detection circuit in aconventional mobile station;

FIG. 14 is a schematic view of a receiving point as it is rotated inphase;

FIG. 15 is a circuit diagram of the second half of the detection circuitin the conventional mobile station;

FIG. 16 is a circuit diagram of an averaging circuit in the second halfof the detection circuit;

FIG. 17 is a circuit diagram of a data delaying unit in the second halfof the detection circuit;

FIG. 18 is a circuit diagram of a phase correction circuit in the secondhalf of the detection circuit; and

FIG. 19 is a schematic view depicting data transmission by theconventional mobile station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention relating to a CDMA mobilecommunication system will now be described in detail with reference tothe accompanying drawings. In FIGS. 1 through 12, the component partswith their functionally identical or equivalent counterparts included inthe prior art examples of FIGS. 13 through 18 are designated by likereference numerals.

First Embodiment

FIG. 1 is a circuit diagram showing the overall constitution of a CDMAmobile communication system practiced as the first embodiment of theinvention. The base station, which transmits data usually to a pluralityof mobile stations, is shown sending data to a single mobile station inFIG. 1. In the left-hand half of FIG. 1, reference numeral 1 is a basestation; 51 is a modulation circuit; 91 is a spread code generator; 54is a radio frequency quadrature modulator; 61 is a temperaturecompensated crystal oscillator; 58 is a circulator for separating atransmitted radio signal from a received radio signal; 12 is the firsthalf of a detection circuit; 62 is a radio frequency quadraturedemodulator; and 64 is a low-pass filter. Reference characters I and Qare an in-phase component signal and a quadrature signal respectively;PN_(−I D) and PN_(−Q D) are spread code signals for the signals I and Qrespectively; S₁ ₁ and S_(Q 1) are an in-phase component signal and anopposite-phase component signal, respectively, subject to the spreadingof the output of the detection circuit first half 12; C_(B) is a carrieroutput by the oscillator 61; and 55 is an air transmission channel, Inthe right-hand half of FIG. 1, reference numeral 2 is a mobile station;52 is the first half of a detection circuit; 59 is a circulator forseparating a received radio signal from a transmitted radio signal; 57is a radio frequency quadrature demodulator; 56 is a low-pass filter;and 63 is a voltage-controlled oscillator. Reference characters C_(M)stand for a carrier output by the oscillator 63, and AFC for a controlsignal for controlling the frequency of the oscillator 63. Referencenumeral 70 denotes a frequency controller for generating the controlsignal AFC. Reference characters CS_(S) and SN_(S) stand for phasecorrection signals of the first step, to be described later; and S_(I)and S_(Q) for an in-phase component signal and an opposite-phasecomponent signal subject to the spreading of the output of the detectioncircuit first half 52. Reference numeral 11 is a demodulation circuit;25 is a spread code generator; and 66 is a radio frequency quadraturemodulator. Reference characters I₁ and Q₁ represent an in-phasecomponent signal and a quadrature component signal, respectively, of thedata transmitted by the mobile station to the base station; andPN_(−I D) and PN_(−Q D) denote spread code signals for the signals I₁and Q₁, respectively, output by the generator 25.

Described below is the case in which the base station 1 transmits dataand a pilot signal over a down link to the mobile station 2. Themodulation circuit 51 is substantially the same in constitution as itsconventional counterpart in FIG. 13. The data to be transmitted passesthrough a QPSK modulator, not shown, to become signals I and Q. Thesignals I and Q are spread by use of the spread code signals PN_(−I D)and PN_(−Q D).

The spread signals are turned by the radio frequency quadraturemodulator 54 into mutually perpendicular radio frequency band signalsthat are transmitted from an antenna past the circulator 58. Thetemperature compensated crystal oscillator 61 supplies the modulator 54with the carrier C_(B).

Although not shown, An in-phase signal I_(P) and a quadrature signalQ_(P) of the pilot signals are spread respectively by spread codesignals PN_(−I P) and PN_(−Q P). Both spread code signals have the samechip rate as in the case of data. The pilot signals thus spread aresubject to radio frequency quadrature modulation by the same carrierC_(B) as with data. Following the modulation, the signals turn intomutually perpendicular signals transmitted on the same radio frequencyband as with data.

FIG. 2 schematically shows a radio frequency band signal transmitted bythe base station 1. In FIG. 2, reference numeral 92 is a radio frequencyband signal representing the pilot signal, and 93 is a radio frequencyband signal that carries data. Data 2 in the signal 93 is destined tothe mobile station 52; data 1 and p are directed to other mobilestations. The data signals 1 and p are each spread by a different spreadcode signal.

The data and pilot signals are thus transmitted on the same radiofrequency band and received by the mobile station 2 (right-hand half ofFIG. 1). The received signals are fed to the radio frequency quadraturedemodulator 57 past the circulator 59. The output of the demodulator 57,from which the low-pass filter 56 removes the spurious part, becomes thesignals S_(I) and S_(Q). The voltage-controlled oscillator 63 suppliesthe demodulator 57 with the carrier C_(M).

The signals S_(I) and S_(Q) are despread and phase-corrected by thesecond half of the detection circuit. This yields restored signals I_(R)and Q_(R) originating from the initial signals I and Q. FIG. 3 is acircuit diagram of the second half of the detection circuit in themobile station. The output terminals of the accumulator 41 are connectedto the input terminals of the frequency controller 70 which is fedthereby with the phase correction signals CS_(S) and SN_(S) of the firststep. Except for these connections, the setup of FIG. 3 is the same asthat of the conventional circuit in FIG. 15. The component parts havingtheir functionally identical or equivalent counterparts included in theprior art examples will not be described further.

The oscillator 63 (in the right-hand half of FIG. 1) is a known circuitusing a variable capacitance diode (not shown) as the element todetermine the oscillation frequency. The diode has its capacitancechanged when fed with the control signal AFC, whereby the oscillationfrequency is controlled.

The frequency controller 70 that outputs the control signal AFC works asfollows: a phase shift of Δφ is detected as a phase change of aboutone-symbol period stemming from the frequency error of the oscillator63. The sine component (sin Δφ) of the phase shift is fed to anintegrator so that the latter will output the control signal AFC. FIG. 4shows the circuit constitution of the frequency controller 70. In FIG.4, reference numerals 700 and 701 are delay gates (Ds) having a delaytime of one-symbol period each, 705 and 706 are multipliers, 707 is asubtracter, 708 is a multiplier, and 709 is an integrator.

The signals CS_(S) and SN_(S) are delayed by the delay gates 700 and701. The multiplier 706 multiplies the signal SN_(S) by a signalsucceeding the signal CS_(S) by one symbol. The multiplier 705multiplies the signal CS_(S) by a signal succeeding the signal SN_(S) byone symbol. The subtracter 707 subtracts the product of the multiplier706 from that of the multiplier 705, yielding an error signal SNΔ havinga value of sin Δφ. If Δφ<<π, then sin Δφ is approximately equal to Δφ.The error signal SNΔ having the value of sin Δφ is multiplied by themultiplier 708 to provide a predetermined loop gain. The multipliedresult is integrated by the integrator 709 that produces the controlsignal AFC.

The controller 70, oscillator 63 and radio. frequency quadraturedemodulator 57 in FIG. 1 as well as the despreading unit 21 andaccumulator 41 in FIG. 3 constitute a control loop in which theintegrator 709 integrates the signal SNΔ so that the latter willapproach zero. This arrangement inhibits the frequency error and keepsthe frequency of the oscillator in the mobile station as accurate asthat of the oscillator in the base station.

The phase change Δφ is also caused by the phase error attributable tofading. However, the fading-triggered phase change is generally veryslow and thus quite small compared with the change caused by frequencyerror. For a period of one symbol or thereabout, there is practically noharm in assuming that the change Δφ is caused solely by frequency error.

The example explained above is one in which the processing of thecontroller 70 is carried out in a one-symbol period. If the frequencyerror is very small during the one-symbol period, it is possible toperform the processing of the controller 70 over a period involving aplurality of consecutive symbols. In this case, the period must notexceed the average delay time T for the averaging circuit 43 (FIG. 16).

Conversely, if the frequency error is relatively large during theone-symbol period, the processing needs to be carried out at a speedhigher than the symbol rate. FIG. 5 shows a circuit diagram of analternative frequency controller 70 performing its processing morequickly than the symbol rate. In FIG. 5, reference numerals 710 and 711are abstract code circuits, 712 and 713 are delay gates with their delaytime shorter than the one-symbol period, 714 and 715 are exclusive-ORgates, and 718 is an integral calculus. The abstract code circuits 710and 711 extract the signs (plus or minus) from the signals CS_(S) andSN_(S) respectively. The extracted signs indicate a quadratic movementof the pilot signal coordinates caused by the phase shift φ, as shown inFIG. 14. For example, if the phase shift φ falls within a range of 180through 270 degrees, the receiving point moves into the third quadrant,and the signals CS_(S) and SN_(S) have the minus signs. The abstractcode circuits 710 and 711 recognize the absence of frequency error (flag“0”) if the signals have the plus signs, or the presence of frequencyerror (flag “1”) if the signals have the minus signs. The flags “0” and“1” are output as sign signals “cos-flag” and “sin-flag” respectively.

The sign signal “cos-flag” and the sign signal “sin-flag” that haspassed the delay gate 713 are fed to the gate 714. The sign signal“sin-flag” and the sign signal “cos-flag” that has passed the delay gate712 are supplied to the gate 715. The output signals of the gates 714and 715 are sent to the integral calculus 718. If the gate 714 outputs“1”, then the integrator 718 outputs as the control signal AFC a voltagethat raises the frequency of the oscillator 63; if the gate 715 outputs“1”, the integrator 718 outputs as the control signal AFC a voltage thatlowers the reference frequency. Where the processing needs to beperformed faster than the symbol rate, as in this example, it ispossible to implement a high-speed frequency controller that dispenseswith multipliers carrying out time-consuming multiplications.

The voltage-controlled oscillator 63 and the two kinds of frequencycontroller 70 may each be constituted by a known semiconductorintegrated circuit. Thus constituted, the inventive setup isincorporated advantageously in mobile stations for use by general users.

Returning to FIG. 1, what follows is a description of the case in whichthe mobile station 2 transmits data and pilot signals over an up link tothe base station 1. The data to be transmitted undergoes QPSK modulation(not shown) to become signals I₁ and Q₁ (bottom right in FIG. 1). Thesignals I₁ and Q₁ are spread by the spread code signals PN_(−I D) andPN_(−Q D) from the spread code generator 25. The signals thus spreadpass through the radio frequency quadrature modulator 66 to becomemutually perpendicular radio frequency band signals that are transmittedfrom an antenna past the circulator 59. The voltage-controlledoscillator 61 supplies the modulator 66 with the carrier C_(M).

In transmitting the pilot signal to the base station 1, the mobilestation 2 multiplexes the signal with the data on a time-division basis.According to this method, the signals I₁ and Q₁ make up a signal formhaving the data and pilot signals multiplexed therein. The data andpilot signals are both spread by the spread code signals PN_(−I D) andPN_(−Q D). FIG. 6 shows a radio frequency band signal multiplexed in themanner described. In FIG. 6, reference numeral 94 is a pilot signalpart, and 95 is a data part.

The signal received by the antenna of the base station 1 is sent to theradio frequency quadrature demodulator 62 past the circulator 58 in thefirst half 51 of the detection circuit (bottom left in FIG. 1). Theoutput signal of the demodulator 62, from which the low-pass filter 64removes the spurious part, turns into signals S_(I 1) and S_(Q 1). Thedemodulator 62 is supplied with the carrier C_(B) from the oscillator61. The signals S_(I 1) and S_(Q 1) are subject to despreading and phasecorrection in the second half of the detection circuit, to be describedlater. The despreading and phase correction processes provide thereconstructed signals I_(1 R) and Q_(1 R) originating from the initialsignals I₁ and Q₁.

FIG. 7 is a circuit diagram of the second half of the detection circuitin the base station 1. In FIG. 7, reference numeral 80 is a receivedsignal despreading unit; 91 is a spread code generator; 82 is anaccumulator; 83 is a phase correction signal extracting unit thatextracts phase correction signals CS_(S 1) and SN_(S 1) of the firststep; 84 is an averaging circuit that receives the signals CS_(S 1) andSN_(S 1) from the extracting unit 83 and outputs phase correctionsignals CS₁ and SN₁; 85 is a data extracting unit that extracts the datapart from the signal converted to the symbol rate; 103 is a datadelaying unit that delays the extracted data by the average delay timeof the averaging circuit 84; and 88 is a phase correction circuit thatrotates in phase the data from the delaying unit 103 and outputs thesignals I_(1 R) and Q_(1 R).

The received signal despreading unit 80 despreads each of the receivedsignals S_(I 1) and S_(Q 1) using the two spread code signals PN_(−I D)and PN_(−Q D) from the spread code generator 91. The four chip ratesignals thus obtained are converted by the accumulator 82 into symbolrate signals A₁ through A₄. The phase correction signal extracting unit83 is supplied with the sum of the signals A₁ and A₄ (including thecosine component of the pilot signal) on the one hand, and with thedifference between the signals A₃ and A₂ (including the sine componentof the pilot signal) on the other. The extracting unit 83 extracts onlythe pilot signal part from the time-division multiplexed signals so asto output the phase correction signals CS_(S 1) and SN_(S 1) of thefirst step. The averaging circuit 84 averages a plurality of symbols ofthe signals CS_(S 1) and SN_(S 1) to output the phase correction signalsCS₁ and SN₁ for use in data phase rotation.

The signals A₁ through A₄ are also sent to the data extracting unit 85.The extracting unit 85 extracts only the data part from thetime-division multiplexed signals. The four-signal data thus obtained isforwarded to the data delaying unit 103. The delaying unit 103 delayseach of the received four signals and outputs data D₁ ₀ ₁ through D₄ ₀₁. The circuit constitution of the phase correction circuit 88 is thesame as that shown in FIG. 17.

With the first embodiment, the values of phase rotation by thecorrection signals CS₁ and SN₁ are set as indicated below. FIG. 8 showsthe received signal structured in units of symbols. In FIG. 8, a pilotsignal of h symbols and a data signal of j symbols are alternatelyreceived. Initially, the averaging circuit 84 averages the h symbols ofa pilot signal 98 and the h symbols of a pilot signal 100. The averagingoperation determines phase rotation quantities of φh1 and φh2. Theamount of phase rotation per symbol of data 99 is given as

φh 1(1−s/h)+φh 2(s/h)

where s stands for the s-th symbol (s=1−j). In this manner, the phaserotation is accomplished while the pilot signals preceding andsucceeding the data part are taken into consideration. This requiresdelaying the current data until the ensuing pilot signal is received.Thus the average delay time, i.e., the delay time of the delaying unit103, is determined as the j-symbol period of the data 99 supplemented bythe h-symbol period of the pilot signal 100.

Where the up link described above is in effect, the radio frequencyquadrature modulator 66 (bottom right in FIG. 1) of the mobile station 2is supplied with the carrier C_(M) output and kept precise by thevoltage-controlled oscillator 63. This allows the base station 1 toavoid the problem of frequency error and to implement stable detection.That in turn makes it possible to adopt a spreading circuit that keepsthe chip rate of the spread code constant where the mobile stationtransmits data at a bit rate lower than the standard rate. If k isassumed to represent the spreading ratio in effect when the data bitrate is standard, the spreading ratio is changed to bk where the bitrate is 1/b (b≧1) of the standard bit rate.

FIG. 9 shows transmitted signals of different bit rates. In FIG. 9, theaxis of abscissa represents time and the axis of ordinate denotestransmission power.

Reference numeral 160 is a signal that transmits data at the standardbit rate with a spreading ratio of k; 161 is a signal that transmitsdata at ½ of the standard bit rate with a spreading ratio of 2k, poweredby ½ of the power level for the standard bit rate; 162 is a signal thattransmits data at ¼ of the standard bit rate with a spreading ratio of4k, powered by ¼ of the standard power level; and 163 is a signal thattransmits data at ⅛ of the standard bit rate with a spreading ratio of8k, powered by ⅛ of the standard power level. In transmitting data atsuch different bit rates, the first embodiment implements CDMAcommunication by varying the circuit constant in keeping with the bitrate but without changes in the circuit constitution.

Second Embodiment

Described below is the second embodiment of the inventive CDMA mobilecommunication system in which a plurality of mobile stations areassigned different spread codes for their pilot signals, each mobilestation transmitting the pilot signal using the assigned spread codeover an up link to the base station. Data is transmitted by use of themodulation circuit 11 shown in the right-hand half of FIG. 1. Althoughnot shown, an in-phase signal and a quadrature signal, of the pilotsignals are spread respectively by spread code signals having the samechip rate as in the case of data. The pilot signals thus spread aresubject to radio frequency quadrature modulation by the same carrierC_(B) as with data. Having undergone the modulation, the signals turninto mutually perpendicular signals transmitted on the same radiofrequency band as with data.

FIG. 10 schematically shows radio frequency band signals transmitted bythe mobile station 2. In FIG. 10, reference numeral 96 is a radiofrequency band pilot signal, and 97 is a radio frequency band datasignal. The pilot signal is transmitted at a power level lower than thedata signal. The transmitted signals are received by the base station 1constituted by the first half of the detection circuit 12 in the bottomleft portion of FIG. 1 and by a circuit having the same construction asthe second half of the detection circuit in FIG. 3.

The modulation circuit 11 in the mobile station 2 utilizes the carrierC_(M) kept precise for radio frequency quadrature modulation. Thisallows the base station 1 to circumvent the problem of frequency errorand to implement stable detection.

Third Embodiment

Described below is the third embodiment of the inventive CDMA mobilecommunication system which derives the phase correction signals of thefirst step from the phase rotation changes of data, with no use of pilotsignals for frequency control. With the third embodiment, the data to betransmitted from the base station 1 is subject to BPSK (binary phaseshift keying) modulation. Signals I_(B) and Q_(B) are acquired throughthe BPSK modulation. The modulation circuit of the base station 1 andthe first half of the detection circuit in the mobile station 2 inconnection with the signals I_(B) and Q_(B) are the same as those shownin FIG. 1. The second half of the detection circuit in the mobilestation 2 is illustrated in FIG. 11. In FIG. 11, reference numeral 45represents a temporary judge circuit. Reference characters CS_(C B) andSN_(C B) denote input signals to the temporary judge circuit 45, andCS_(S B) and SN_(S B) indicate phase correction signals of the firststep output by the temporary judge circuit 45.

The data despreading unit 42, spread code generator 25, accumulator 44,averaging circuit 43, data delaying unit 48, phase correction circuit 49and frequency controller 70 in FIG. 11 are the same in function as theircounterparts of the first embodiment in FIGS. 3 and 4. The signalsCS_(S B) and SN_(S B) are supplied to the frequency controller 70generating the control signal AFC for the voltage-controlled oscillator63 (FIG. 1). The signals CS_(S B) and SN_(S B) are also fed to theaveraging circuit 43 that generates phase correction signal CS_(B) andSN_(B).

In the second half of the detection circuit in the mobile station 2 ofFIG. 11, the signals S_(I B) and S_(Q B) output by the detection circuitfirst half 52 (right-hand half in FIG. 1) are despread by the datadespreading unit 42 using the spread code signals PN_(−I D) andPN_(−Q D) for the signals I_(B) and Q_(B) respectively. The despreadsignals are converted by the accumulator 44 from the chip rates tosymbol rate signals D_(1 B) through D_(4 B). The signals D_(1 B) andD_(4 B) are added up to yield the signal CS_(C B) representing thecosine component of the data, and the signal D_(2 B) is subtracted fromthe signal D_(3 B) to give the signal SN_(C B) representing the sinecomponent of the data. The signals CS_(C B) and SN_(C B) are fed to thetemporary judge circuit 45.

The data is composed of “1” and “0” iterations or of no changes persymbol (the pilot signal remains unchanged). Thus where the signalsCS_(C B) and SN_(C B) are both inverted in phase per symbol due to datachanges, it is desired to generate signals that would correct the phaseinversion so as to render the input signals apparently unchanged with noshift in phase. Such signals, when generated by the temporary judgecircuit 45, serve as phase correction signals of the first stepfunctionally equivalent to those acquired by use of the pilot signal.

FIG. 12 is a circuit diagram of the temporary judge circuit 45. In FIG.12, reference numerals 182, 183 and 189 are delay gates (Ds) having adelay time of one-symbol period each; 184 and 185 are multipliers; 180is an adder; 186 is a abstract code circuit; 181 is an exclusive-ORgate; and 187 and 188 are sign inverting units.

The signal CS_(C B) is multiplied by a signal preceding the signalCS_(C B) by one symbol, and the product is fed to the adder 180. At thesame time, the signal SN_(C B) is multiplied by a signal preceding thesignal SN_(C B) by one symbol, and the product is supplied to the adder180. The result of the addition is sent to the abstract code circuit 186which outputs a signal indicating whether the signals CS_(C B) andSN_(C B) are simultaneously inverted in phase.

The output signal of the extracting unit 186 is sent to the exclusive-ORgate 181. The other input of the exclusive-OR gate 181 is a signalpreceding by one symbol the output signal of the same gate. Theexclusive-OR gate 181 outputs “1” if the absence of the simultaneousphase inversion preceding a given symbol is replaced by the presence ofthe inversion following that symbol or vice versa; the exclusive-OR gate181 outputs “0” if the simultaneous phase inversion is either absent orpresent both before and after a symbol (if the simultaneous phaseinversion of the signals CS_(C B) and SN_(C B) continues before andafter a symbol, that means the original data is restored). With theexclusive-OR gate 181 outputting “1”, the sign inverting units 187 and188 output the input signals CS_(C B) and SN_(C B) after simultaneouslyinverting them in phase. Where the exclusive-OR gate 181 outputs “0”,the input signals CS_(C B) and SN_(C B) are output uninverted. Theprocess above turns the signals CS_(C B) and SN_(C B) into the phasecorrection signals CS_(S B) and SN_(S B) of the first step respectively.

The constitution and the workings of the frequency controller 70 are thesame as those of the first and the second embodiments. Given the phasecorrection signals CS_(S B) and SN_(S B) of the first step, thefrequency controller 70 outputs the control signal AFC to control thevoltage-controlled oscillator 63. With the third embodiment, the phaserotation of the data following despreading is corrected and thevoltage-controlled oscillator 63 is kept accurate as effectively as inthe case where the pilot signal is utilized. The third embodiment thuspermits the base station 1 and mobile station 2 to implement stabledetection. In particular, the mobile station 2 is allowed to realizedata transmission with an appropriate spreading ratio selected.

Although the first through the third embodiments adopt QPSK or BPSKmodulation upstream of the spreading process, this is not limitative ofthe invention. The invention is not dependent on the pre-spreadmodulation scheme because the invention aims to keep precise the carrierfor radio frequency modulation and demodulation. Any system ofpre-spreading modulation may be adopted in conjunction with theinvention. The invention, when suitably embodied, promises stableoperation in both coherent detection and differential detection.

According to the invention, the pilot signal acquired from despreadingis used to detect frequency error, and the frequency of the carrier iscontrolled so as to reduce the detected frequency error to zero. Thisallows the mobile station to implement stable detection with a minimumof bit error. Since the same carrier is used in radio frequencyquadrature modulation, the base station is allowed to realize stabledetection with reduced bit error. When the mobile station is to transmitdata at a low bit rate, an appropriate spreading ratio may be selectedin accordance with the bit rate. This arrangement averts the process ofkeeping the spreading ratio constant—a process that complicatescircuitry. The features above make it possible to implement a morepractical CDMA mobile communication system of higher performance thanever before.

It is further understood by those skilled in the art that the foregoingdescription pertains to preferred embodiments of the disclosed systemand that various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

What is claimed is:
 1. A mobile station for use with a mobilecommunication system multiplexing a plurality of communication channelsusing spread spectrum codes comprising: an oscillator; a radio frequencydemodulator demodulating a radio frequency band signal transmitted froma base station by use of a signal from the oscillator; and a despreadingcircuit extracting a pilot signal by despreading the demodulated radiofrequency band signal using the spread spectrum code assigned to thepilot signal; wherein an oscillation frequency of the oscillator iscontrolled by use of a change of a phase shift detected using a signalrepresenting a phase shift detected from the extracted pilot signal. 2.A mobile station according to claim 1, further comprising: anaccumulator converting the extracted pilot signal with a chip rate bythe despreading circuit into a pilot signal with a symbol rate; whereina phase shift detected from the pilot signal is a phase shift with asymbol rate outputted by the accumulator.
 3. A mobile station accordingto claim 2, wherein the oscillation frequency of the oscillator iscontrolled by use of the phase shift detected with a symbol rate.
 4. Amobile station according to claim 2, wherein the oscillation frequencyof the oscillator is controlled by use of an average of the phase shiftdetected with a symbol rate during a set period.
 5. A mobile stationaccording to claim 1, wherein the oscillator is a voltage-controlledoscillator.
 6. A base station for use with a mobile communication systemmultiplexing a plurality of communication channels using spread spectrumcodes comprising; a spreading circuit spreading a pilot signal using thespread spectrum code assigned to the pilot signal; an oscillator; aradio frequency modulator modulating the spread pilot signal into aradio frequency band signal by use of an oscillation frequency of theoscillator; and a radio frequency demodulator demodulating a radiofrequency band signal transmitted from a mobile station by use of anoscillation frequency of the oscillator; wherein the radio frequencyband signal transmitted from a mobile station is a signal modulated witha carrier controlled by use of a phase shift detected from the pilotsignal.
 7. A base station according to claim 6, wherein the phase shiftis detected with a symbol rate.
 8. A mobile station for use with amobile communication system multiplexing a plurality of communicationchannels using spread spectrum codes comprising: an oscillator; a radiofrequency demodulator demodulating a radio frequency band signaltransmitted from a base station by use of a signal form the oscillator;a first despreading circuit extracting a data signal by despreading thedemodulated radio frequency band signal using the spread spectrum codeassigned to the data signal; a second despreading circuit extracting apilot signal by despreading the demodulated radio frequency band signalusing the spread spectrum code assigned to the pilot signal; and a phasecorrection circuit performing phase correction for the data signal byuse of a phase shift detected from the extracted pilot signal; whereinan oscillation frequency of the oscillator is controlled by use of asignal representing the phase shift.
 9. A mobile station according toclaim 8, further comprising: an accumulator converting the extractedpilot signal with a chip rate by the second despreading circuit into apilot signal with a symbol rate; wherein a phase shift detected from thepilot signal is a phase shift with a symbol rate outputted by theaccumulator.
 10. A mobile station according to claim 9, furthercomprising: an averaging circuit averaging the phase shift with a symbolrate during a first period; wherein the phase correction circuitperforms phase correction for the data signal by use of the averagedphase shift.
 11. A mobile station according to claim 9, wherein theoscillation frequency of the oscillator is controlled by use of thephase of the phase shift detected with a symbol rate.
 12. A mobilestation according to claim 10, wherein the oscillation frequency of theoscillator is controlled by use of an average of the phase of the phaseshift detected with a symbol rate during a second period that does notexceed the first period.
 13. A mobile station according to claim 8,wherein the oscillator is a voltage-controlled oscillator.